Biological nitrogen fixation can be an important source of nitrogen in tropical forests that serve as a major CO₂ sink. Extensive deforestation of the Amazon is known to influence microbial communities and the biogeochemical cycles they mediate. However, it is unknown how diazotrophs (nitrogen–fixing microorganisms) respond to deforestation and subsequent ecosystem conversion to agriculture, or whether they can recover in secondary forests that establish after agriculture is abandoned. To address these knowledge gaps, we combined a spatially-explicit sampling approach with high-throughput sequencing of nifH genes. The main objectives were; to assess the functional distance decay relationship of the diazotrophic bacterial community in a tropical forest ecosystem and secondly to quantify the role of various factors that drive the observed changes in the diazotrophic community structure. We observed an increase in local diazotrophic diversity (α-diversity) with a decrease in community turnover (β-diversity), associated with a shift in diazotrophic community structure with the forests-to-pasture conversion. Both diazotrophic community turnover and structure showed signs of recovery in secondary forests. Changes in the diazotrophic community were primarily driven by the change in land use rather than differences in geochemical characteristics or geographic distances. The diazotroph communities in secondary forests resembled those in primary forests, suggesting that at least partial recovery of diazotrophs is possible following agricultural abandonment.

IMPORTANCE

The Amazon is a major tropical forest that is being deforested at an alarming rate to create space for cattle ranching and agriculture. Diazotrophs (nitrogen-fixing microorganisms) play an important role in supplying soil N for plant growth in tropical forests. It is unknown how diazotrophs respond to deforestation and whether they can recover in secondary forests that establish after agriculture is abandoned. Using high-throughput sequencing of nifH genes we described the response of diazotrophs β-diversity and identified major drivers of changes in diazotrophs from forest to pasture and from pasture to secondary forest conversion. Studying the impact of land-use change on diazotrophs is important for a better understanding of the impact of deforestation on tropical forest ecosystem functioning and our results on the potential recovery of diazotrophs in secondary forest imply the possible restoration of ecosystem functions in secondary forests.

生物固氮可能是充当主要CO ₂汇的热带森林中氮的重要来源。众所周知，亚马逊地区广泛的森林砍伐会影响微生物群落及其介导的生物地球化学循环。但是，未知重氮菌（固氮微生物）如何应对森林砍伐和随后的生态系统向农业的转化，或者它们能否在废弃农业后建立的次生森林中恢复。为了解决这些知识差距，我们将空间明确的采样方法与nifH的高通量测序相结合基因。主要目标是；评估热带森林生态系统中重氮营养菌群落的功能距离衰减关系，其次要量化驱动观察到的重氮营养菌群落结构变化的各种因素的作用。我们观察到局部重氮营养型多样性（α-多样性）的增加与群落更新（β-多样性）的减少有关，这与森林到牧草转化的重氮营养型群落结构发生了变化。重氮营养群落的更新和结构都显示出次生林地恢复的迹象。重氮营养群落的变化主要是由土地用途的变化而不是地球化学特征或地理距离的差异引起的。次生林中的重氮营养族与原始林中的相似，

重要性

亚马逊河是主要的热带森林，正在以惊人的速度砍伐森林，以创造牧场和农业用地。重氮菌（固氮微生物）在为热带森林中的植物生长提供土壤氮素方面发挥着重要作用。未知重氮菌对森林砍伐的反应以及它们是否可以在废弃农业后建立的次生林中恢复。使用nifH的高通量测序我们描述了重氮营养生物β多样性的响应基因，并确定了重氮营养生物从森林到牧场以及从牧场到次生林转换的主要驱动力。研究土地利用变化对重氮菌的影响对于更好地了解毁林对热带森林生态系统功能的影响非常重要，我们对次生森林中重氮菌潜在恢复的研究结果表明，次生森林中生态系统功能可能会恢复。